Molecular Orientation of Carboxylate Anions at the Water-Air Interface Studied with Heterodyne-Detected Vibrational Sum-Frequency Generation

Analysis of the mechanism of difference in umami peptides from oysters (Crassostrea ariakensis) prepared by trypsin hydrolysis and boiling through hydrogen bond interactions

This study compares umami peptides prepared by trypsin hydrolysis and boiling and analyzes their umami intensity and characteristics. Using a taste reconstitution model and taste evaluation analysis, the study revealed that umami peptides prepared by boiling have a higher umami contribution. Myosin and heat shock protein were identified as marker proteins for revealing differences of cleavage sites. Boiling releases a higher proportion of acidic amino acids at the protein cleavage sites p1-p1', whereas trypsin hydrolysis releases more basic amino acids. Molecular docking simulation and electrostatic potential observation showed that acidic amino acid residues have a wider binding range with the umami receptor T1R1-VFT. Acidic amino acids lower the isoelectric point (pI) of umami peptides, enhancing their negative charge at pH 7, which are more likely to bind to the positively charged regions of the umami receptor.

Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces

Aqueous glycine plays many different roles in living systems, from being a building block for proteins to being a neurotransmitter. To better understand its fundamental behavior, we study glycine's orientational behavior near model aqueous interfaces, in the absence and presence of electric fields and biorelevant ions. To this purpose, we use a surface-specific technique called heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Using HD-VSFG, we directly probe the symmetric and antisymmetric stretching vibrations of the carboxylate group of zwitterionic glycine. From their relative amplitudes, we infer the zwitterion's orientation near surfactant-covered interfaces and find that it is governed by both electrostatic and surfactant-specific interactions. By introducing additional ions, we observe that the net orientation is altered by the enhanced ionic strength, indicating a change in the balance of the electrostatic and surfactant-specific interactions.

Determining the Surface pKa of Perfluorooctanoic Acid

Perfluorooctanoic acid (PFOA) is an environmentally prevalent and persistent organic pollutant with toxic and bioaccumulative properties. Despite the known importance of perfluorinated pollutants in the global environment, molecular-level details of the physicochemical behavior of PFOA on aqueous interfaces remain poorly understood. Here, we utilized two surface-specific techniques, vibrational sum frequency generation spectroscopy (SFG) and surface tensiometry, to investigate the pH-induced structural changes of PFOA and octanoic acid (OA) and determined the apparent pKa at the air-water surface. The SFG spectra and surface activity model were investigated over a wide range of pHs. With the surface tension measurements, the surface pKa values for OA and PFOA are determined to be 3.8 ± 0.1 and 2.2 ± 0.2, respectively. These results could provide insights into improved remediation of PFOAs and may impact climate modeling of perfluorinated alkyl chain molecules.